The human nasal mucosa is an accessible and clinically important model for the study of microbial interactions with host defenses. Nasal carriage of Staphylococcus aureus (SA) is the most common clinical disorder of mucosal host defense but its molecular and cellular basis is not understood. The disorder is of increasing clinical importance because nosocomial infections are commonly spread by nasal carriers of methicillin-resistant SA and other SA strains increasingly resistant to antibiotics. Our evidence indicates that colonization may be due to impaired innate antimicrobial activity of nasal fluid. The current proposal aims to identify the underlying cause(s) of SA carriage by examining protein determinants of SA carrier airways colonization and comparing microbicidal components of SA carrier fluid to nasal fluids from donors that are not colonized with SA. We hypothesize that 1) noncarriers express antimicrobial (poly)peptide factors some of which are either lacking or defective in SA carrier fluid, 2) altered or deficient host defense factors in airways fluid contribute to the progressive colonization of SA in carriers, and 3) correcting the dysregulated components will restore the antimicrobial activity of SA carrier airway fluid against isolates of SA. To test these hypotheses, we will: 1) detect cationic (poly)peptides that are differentially expressed between nasal fluid from donors carrying SA and fluid from donors that are not colonized with SA utilizing a novel two-dimensional gel electrophoresis (2-DE) approach, 2) identify (poly)peptides that are differentially expressed between donors carrying SA and donors that are not colonized with SA, and 3) reconstitute the antimicrobial activity of SA carrier nasal fluid by replacing abnormal or missing (poly)peptides with intact counterparts purified from noncarrier fluid. Our proposed studies represent a novel approach to identify and link human airway disease biomarkers (cationic polypeptide antimicrobials) with their effects (SA nasal carriage). Together, these studies will for the first time characterize the host resistance factors for a common and increasingly important source of nosocomial infection, and will develop a neglected but very useful natural model for the study of the interactions of bacteria with an accessible mucosal surface.